linux-kernel - Re: [PATCH 5/7] watchdog: dw_wdt: Support devices with asynch clocks

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Message-ID: <158681117129.84447.14839907555361565766@swboyd.mtv.corp.google.com>
Date:   Mon, 13 Apr 2020 13:52:51 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Sergey Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/7] watchdog: dw_wdt: Support devices with asynch clocks

Quoting Sergey Semin (2020-04-10 11:59:34)
> Michael, Stephen, could you take a look at the issue we've got here?
> 
> Guenter, my comment is below.
> 
> On Sun, Mar 15, 2020 at 07:22:07AM -0700, Guenter Roeck wrote:
> > On Fri, Mar 06, 2020 at 04:27:45PM +0300, Sergey.Semin@...kalelectronics.ru wrote:
> > > @@ -358,10 +375,27 @@ static int dw_wdt_drv_probe(struct platform_device *pdev)
> > >             goto out_disable_clk;
> > >     }
> > >  
> > > +   /*
> > > +    * Request APB clocks if device is configured with async clocks mode.
> > > +    * In this case both tclk and pclk clocks are supposed to be specified.
> > > +    * Alas we can't know for sure whether async mode was really activated,
> > > +    * so the pclk reference is left optional. If it it's failed to be
> > > +    * found we consider the device configured in synchronous clocks mode.
> > > +    */
> > > +   dw_wdt->pclk = devm_clk_get_optional(dev, "pclk");
> > > +   if (IS_ERR(dw_wdt->pclk)) {
> > > +           ret = PTR_ERR(dw_wdt->pclk);
> > > +           goto out_disable_clk;
> > > +   }
> > > +
> > > +   ret = clk_prepare_enable(dw_wdt->pclk);
> > 
> > Not every implementation of clk_enable() checks for a NULL parameter.
> > Some return an error. This can not be trusted to work on all platforms /
> > architectures.
> 
> Hm, this was unexpected twist. I've submitted not a single patch with optional
> clock API usage. It was first time I've got a comment like this, that the
> API isn't cross-platform. As I see it this isn't the patch problem, but the
> platforms/common clock bug. The platforms code must have been submitted before
> the optional clock API was introduced or the API hasn't been properly
> implemented or we don't understand something.
> 
> Stephen, Michael could you clarify the situation with the
> cross-platformness of the optional clock API.
> 

NULL is a valid clk to return from clk_get(). And the documentation of
clk_enable() says that "If the clock can not be enabled/disabled, this
should return success". Given that a NULL pointer can't do much of
anything I think any platform that returns an error in this situation is
deviating from the documentation of the clk API.

Does any platform that uses this driver use one of these non-common clk
framework implementations? All of this may not matter if they all use
the CCF.